160 



Fishery Bulletin 100(2) 



16 24"C 



16 24'C 



8 16 24'C 



1400 



2200 



0600 



1400 



2200 



0600 



16 24 "C 



Figure 5 



Vertical movements of four juvenile bluefin tuna: (A) = fish 1, (B) = fish 2, (C) = 

 fish 3, (D) = fish 5). Bottom topography is shown by the shaded area, and 

 the thick horizontal solid bars indicate nighttime. Depth-temperature profiles 

 (mean ±SEM), recorded during each track by using an expendable bathythermo- 

 graph iXBTl system, are shown to the right of each vertical movement plot. 



skipjack tuna (Katsuwonus pelamis). In the Pacific, the 

 latter two species often spend daylight hours associated 

 with reef drop-offs, banks, or man-made fish aggregating 

 devices (FADs), repeatedly move up to =5 nmi away at 

 night, then return to the same area the following day 

 (Yuen, 1970; Holland et al, 1990; Marsac and Cayre, 1998; 

 Dagorn et al., 2000a). The continental shelf where the 

 juvenile bluefin tuna were followed is. however, relatively 

 flat compared with the steep topography around the oce- 

 anic islands, where many of the yellowfin and skipjack 

 tuna were followed. Unlike the situation in the Pacific, the 

 small topographic features on the mid-Atlantic continen- 

 tal shelf probably do not possess sufficient magnetic sig- 



natures that could be detected by the fish (Walker, 1984; 

 Klimley, 1993). Moreover, the movements of the juvenile 

 that we observed closely resembled those of bluefin tuna 

 tracked in the eastern Pacific when they were not near the 

 coast or any apparent significant geological features ( Mar- 

 cineketal,,2001) 



Juvenile bluefin tuna clearly use the entire water col- 

 umn and frequently interact with the bottom when over 

 the continental shelf Similar behaviors were observed in 

 adult bluefin tuna in the Gulf of Maine (western North 

 Atlantic) (Lutcavage et al, 2000). Frequent vertical move- 

 ments of juvenile bluefin tuna likely reflect feeding be- 

 haviors for capturing sand lance {Ammodytes spp. ), which 



